| The variation characteristics of normal stress and normal closure of fractured granites were studied.The effects of temperature,chemical solution and various initial openings on the evolutions of specific normal stiffness of specimens were revealed through normal load compression test taking into account constant circumferential stiffness boundary conditions.The results provide effective data references for simulating and predicting the closure behavior of fractured rocks during deep underground geothermal mining.The main contents and conclusions are as follows:Normal load compression tests were carried out on intact and fractured granite specimens after temperature treatments ranging from 25 ℃ to 600 ℃ and chemical corrosion treatments by p H = 1,3,7 and 12 for 10,30 and 100 days,respectively.The results show that different from intact specimens and specimens with matched fractures,the stress drop occurs during the normal closure of the specimens with mismatched fractures.Then the stress-strain curves tend to be smooth,and the growth rate increases.The increase in temperature reduces the normal stiffness of the fractured granites under both dry and saturated conditions,which will be most obvious at T = 600 °C.The peak compressive strength and normal stiffness of specimens treated by p H = 1 chemical solution reach the minimum,followed by p H = 3 and p H = 7.Compared to d = 10 days,the deteriorations of mechanical properties of the specimens are more obvious at 30 and100 days.The evolutions of normal stiffness of fractured granites were described and characterized.The results indicate that under the B-B model,the growth rate of normal stiffness increases with the increase in normal stress.Under the hyperbolic model,the normal stiffness is linearly proportional to the normal stress.Based on the experimental data,it is found that the normal stiffness increases linearly and then exponentially with the increase in the normal stress.The relationship between the normal stiffness and the normal strain can be characterized by a quadratic function.Several prediction equations to describe the normal stiffness of fractured rocks with normal strain considering different initial openings,temperatures,and p H chemical solution environments are proposed,and the experimental data has a great correspondence with the predicted results.The failure processes of intact specimens and fractured specimens under different lateral conditions were studied by the discrete element numerical simulation software PFC 3D.The results show that the intact specimens are gradually destroyed from both ends of the specimens to the center,and the failure of the fractured specimens occurs firstly in the center,and then gradually to the both ends.The increase in JRC leads to the larger contact area of the fracture rock,which results in the larger normal stress at the same displacement.For the specimens with unmated fractures,with the increase in JRC,the initial openings at the same rotation angle increase,which leads to a smaller contact area at the same normal displacement,and therefore a smaller normal stress and normal stiffness.When the confining pressure increases from 0 MPa to 20 MPa,the peak compressive strength of the intact specimen increases continuously,and the greater the confining pressure is,the slower the rate decreases.For the specimens with unmated fractures,the increase in the confining pressure causes the more obvious stress fluctuation and the higher value after stress reduction. |
